A Versatile Metathesis Reaction for the Formation of Labile Bonds between Group 13 and Group 15 Atoms

Article abstract of DOI:10.1002/anie.200352332

The heterocycles [{Me₂AIE(SiMe₃)₂} _x] (E = P-Bi) are powerful metathesis reagents in the presence of 4-dimethyl-aminopyridine (dmap), thus allowing the synthesis of previously unknown Group 13/15 compounds. Reactions with [Me₃M-dmap] (M = In, TI) proceed with E(SiMe₃)₂ transfer to form the corresponding heterocycles [{Me₂InBi-(SiMe₃) ₂}₃] and [{Me₂TIE(SiMe₃) ₂}₂] (E = P, As), which were characterized by NMR and MS analysis, and by single-crystal X-ray diffraction.

Full text of DOI:10.1002/anie.200352332

Angewandte
Chemie
Group 13/15 Heterocycles
AVersatile Metathesis Reaction for the Formation
of Labile Bonds between Group 13 and Group 15
Atoms**
Florian Thomas, Stephan Schulz,* Heidi Mansikkamäki,
and Martin Nieger
Dedicated to Professor Peter Jutzi
on the occasion of his 65th birthday
Although Group 13/Group 15 compounds have been
intensely studied for some decades, no generally applicable
ꢀ
synthetic pathway for the formation of M E bonds (M = Al–
Tl; E = N–Bi) is known to date. Dihydrogen- and salt-
elimination reactions as well as metathesis reactions with
organolithium reagents are well-established methods for the
ꢀ
ꢀ
formation of M NR2 and M PR2 bonds, but they are
generally ineffective for the synthesis of compounds contain-
ing the heaviest Group 13/15 elements (M = In, Tl; E = Sb,
Bi). To date, dehydrosilylation reactions represent the most
versatile reaction pathway for the generation of this type of
compound.[1] The activation energy is low, thus allowing the
reaction to be performed at low temperature. Additionally,
the tendency of silyl-substituted compounds of the type
[*] Priv.-Doz. Dr. S. Schulz, Dipl.-Chem. F. Thomas, Dr. M. Nieger
Institut für Anorganische Chemie
Universität Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-73-5327
E-mail: sschulz@uni-bonn.de
MSc H. Mansikkamäki
Department of Chemistry
University of Jyväskylä (Finland)
[**] S. Schulz gratefully acknowledges financial support by the Deut-
schen Forschungsgemeinschaft(DFG), the Fonds der Chemischen
Industrie (FCI), the Bundesministerium für Bildung, Wissenschaft,
Forschung und Technologie (BMBF), and Prof. E. Niecke,
Universität Bonn. F. Thomas thanks the FCI, and H. Mansikkamäki,
of the Academy of Finland for fellowship awards.
Angew. Chem. Int. Ed. 2003, 42, 5641 –5644
DOI: 10.1002/anie.200352332
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5641

Communications
R2ESiMe3 to undergo redox reactions is rather low compared
to that of organolithium compounds, R2ELi. Unfortunately,
ꢀ
this reaction pathway cannot be used for the synthesis of In E
ꢀ
and Tl E compounds because of the instability of the
required dialkylindium hydrides R2InH and dialkylthallium
hydrides R2TlH.[2] Consequently, we became interested in an
alternative route to Group 13/15 compounds, in particular
those compounds that contain the heaviest elements of both
groups. Herein, we report a new metathesis reaction, which is
used for the synthesis of the previously unknown heterocycles
of the type [{Me2ME(SiMe3)2}x].
[{Me2AlBi(SiMe3)2}3][3] reacts with the Lewis acid–base
[4]
ꢀ
adduct [Me3In dmap] (dmap = 4-dimethylaminopyridine)
ꢀ
at ꢀ508C with the formation of [Me3Al dmap] and [Me2In-
Bi(SiMe3)2]3 (1). We believe this reaction to proceed through
ꢀ
the cleavage of the Al Bi heterocycle by the reaction with the
strong Lewis base dmap. The subsequent coordination of
Figure 1. Solid-state structure of 1. Selected bond lengths [Š] and
angles [8]: Bi1-In1 2.907(1), Bi1-In2 2.903(1), Bi2-In2 2.914(1), Bi2-In3
2.911(1), Bi3-In1 2.935(1), Bi3-In3 2.920(1), Bi1-Si1 2.628(2), Bi1-Si2
2.622(2), In1-C1 2.182(7), In1-C2 2.165(8), Bi2-Si3 2.637(2), Bi2-Si4
2.635, In2-C3 2.162(8), In2-C4 2.171(8), Bi3-Si5 2.639(2), Bi3-Si6
2.642(2), In3-C5 2.180(8), In3-C6 2.195(8); In1-Bi1-In2 123.0(1), In2-
Bi2-In3 129.1(1), In1-Bi3-In3 129.2(1), Bi1-In1-Bi3 99.7(1), Bi1-In2-Bi2
99.4(1), Bi2-In3-Bi3 104.2(1), C1-In1-C2 126.9(3), Si1-Bi1-Si2 102.3(1),
C3-In2-C4 125.6(3), Si3-Bi2-Si4 100.1(1), C5-In3-C6 116.6(3), Si5-Bi3-
Si6 101.6(1).
ꢀ
ꢀ
InMe3 results in the formation of [dmap Al(Me2)Bi(SiMe3)2
InMe3],[5] which undergoes fragmentation with Al Bi and In
ꢀ
ꢀ
ꢀ
Me bond breakage to give 1 and [Me3Al dmap] (Scheme 1).
Detailed studies on the reactivity of [{Me2AlE(SiMe3)2}x]
ꢀ
(E = P, As, Sb, Bi) towards [Me3M dmap] (M = Ga, In)
ꢀ
Scheme 1. Synthesis of the six-membered In Bi heterocycle 1.
clearly demonstrated that the described reaction pathway is
generally applicable for the formation of heterocycles of the
type [{Me2ME(SiMe3)2}x] (M = Ga, In; E = P, As, Sb, Bi).[10]
The presence of the Lewis base dmap was found to be
essential, as the reactivity of [{Me2AlE(SiMe3)2}x] is signifi-
cantly increased owing to the initial formation of Lewis base
Compound 1 is heat labile and sensitive towards oxygen
and water. Whereas isolated 1 can be stored in the dark at
ꢀ308C for several weeks, it decomposes in solution at
temperatures above ꢀ308C within minutes with the forma-
tion of Bi2(SiMe3)4, InMe3, and elemental In and Bi.
Consequently, NMR spectra (1H, 13C) were recorded at
ꢀ408C in [D8]toluene. Both spectra show only two resonances
arising from the InMe2 and SiMe3 groups, as observed for the
corresponding AlBi[3] and GaBi heterocycles.[6] The mass
spectrum of 1 only shows decomposition fragments, thus
indicating the lability of 1. Single crystals of 1 were obtained
from a toluene solution at ꢀ608C (Figure 1). Compound 1
crystallizes in the monoclinic space group P21/n (No. 14)[7] and
is isostructural to [{Me2AlBi(SiMe3)2}3][3] and [{Me2GaBi-
ꢀ
stabilized, monomeric [R2E AlR2(dmap)]. This is also
known for organolithium reagents, whose reactivity increases
in the presence of donor solvents (disaggregation and
formation of RLi(donor)).[11] Comparable results have been
reported by Driess et al. on the reactivity of [(iBu)2AlPH2]3,
which was found to be a mild PH2-transfer reagent in
coordinating solvents such as THF.[12]
In an attempt to explore the synthetic potential of this new
metathesis reaction, we focused our interest on the synthesis
of TlE heterocycles [{R2TlER02}x]. To date, heterocycles of this
type are completely unknown owing to the high oxidation
potential and the low thermal resistance of TlIII compounds
and because dialkylthallium hydrides R2TlH are not avail-
able. Our initial studies on the reactions of [{Me2AlE-
(SiMe3)2}2] (E = P, As)[13] with [Me3Tl–dmap][4] resulted in
the formation of the corresponding four-membered TlE
heterocycles [{Me2TlP(SiMe3)2}2] 2 and [{Me2TlAs(SiMe3)2}2]
3 in good yields (Scheme 2).
(SiMe3)2}3].[6] The central structural feature of this first
organometallic In Bi compound is the nonplanar six-
membered In3Bi3 ring. Both the In and the Bi atoms adopt
distorted tetrahedral environments, in which the average
endocyclic Bi-In-Bi bond angle (101.18) is significantly
smaller than the average In-Bi-In bond angle (127.18). The
[8]
ꢀ
ꢀ
In Bi bond lengths range from 2.903(1) to 2.935(1) Š
(av 2.915 Š), which is about 0.15 Š longer than the corre-
ꢀ
sponding M Bi bond lengths in [{Me2AlBi(SiMe3)2}3]
(av 2.774 Š)[3] and [{Me2GaBi(SiMe3)2}3] (av 2.762 Š).[6]
However, this increase in bond length is less pronounced
than was expected because of the increase of the covalent
radii of the In atom (1.44 Š) compared to Al (1.25 Š) and Ga
(1.26 Š) atoms.[9]
ꢀ
Scheme 2. Synthesis of the four-membered Tl E heterocycles 2 (E=P)
and 3 (E=As).
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The presence of a triplet in the 31P NMR spectrum of 2
with a 1JTl,P coupling constant of 2462 Hz proves the formation
stabilized adduct [Tl{o-(Ph2PCH2)C6H4}3] (2.795 Š) as well
as in [(Me3SiCH2)3Tl P(SiMe3)3] (2.992 Š).[16] Compound 3 is
ꢀ
[14]
1
of a Tl P heterocycle. The H and 13C NMR spectra of 2
the first organometallic compound that contains a Tl As
ꢀ
ꢀ
ꢀ
and 3 each show a doublet arising from the TlMe2 groups (2:
bond. Consequently, the Tl As bond lengths (2.743(1),
1
2JTl-H = 317 Hz, 1JTl-C = 2391 Hz; 3: 2JTl-H = 313 Hz, JTl-C
=
2.781(1) Š) can only be compared with the Tl As bond
ꢀ
2304 Hz), whereas the SiMe3 groups only show a singlet.
The mass spectra of 2 and 3 do not show the molecular ion
peaks. The mass spectrum of 2 shows the presence of two
lengths in Zintl anions such as [TaAs4Tl2]5ꢀ (2.74–2.88 Š).[17]
ꢀ
The average Tl C bond lengths (2: 2.204 Š, 3: 2.197 Š) are
[4]
ꢀ
almost the same as those in [Me3Tl dmap] (2.180 Š). The
fragments at m/z 557 [(Me3Si)2PTlP(SiMe3)2]+ and m/z 395
endocyclic E-Tl-E bond angles are smaller than the Tl-E-Tl
bond angles, as is generally observed for four-membered
heterocycles of the type [{Me2ME(SiMe3)2}2].[18] However, the
differences within the thallium heterocycles 2 (P-Tl-P =
+
ꢀ
[MeTlP(SiMe3)2] ) that contain a Tl P bond, whereas the
mass spectrum of 3 only shows decomposition fragments.
Crystals of 2 and 3 suitable for single-crystal X-ray
diffraction analyses were obtained from solutions in toluene
at ꢀ308C (2) and ꢀ608C (3) (Figure 2). Compounds 2 and 3
84.5(1)8 versus Tl-P-Tl = 95.5(1)8) and
3 (As-Tl-As =
83.1(1)8 versus Tl-As-Tl = 96.7(1)8), are significantly more
pronounced than in analogous heterocycles containing the
lighter elements of Group 13.
The synthesis of the heat-labile heterocycles 1–3 provides
evidence for the preparative potential of this new synthetic
pathway and extends the methods available for the formation
of Group 13/Group 15 atom bonds. The driving force of the
described reaction is the formation of the stable Lewis acid–
ꢀ
base adduct [Me3Al dmap]. Because of the excellent reac-
tivity of the starting Al/Group 15 atom heterocycles of the
type [{Me2AlE(SiMe3)2}x] and the mild reaction conditions
(low temperature), this metathesis reaction has a promising
potential for the formation of compounds containing yet
unknown bonds between Group 15 atoms and atoms of other
elements.
Experimental Section
1: A suspension of [{Me2AlBi(SiMe3)2}3] (0.41 g, 0.33 mmol) in
ꢀ
toluene (10 mL) at ꢀ508C was combined with [Me3In dmap]
(0.28 g, 1.0 mmol) dissolved in toluene (5 mL). The resulting brown-
ish suspension was allowed to warm to ꢀ308C over 30 min. There-
after, the suspension was quickly warmed until a clear solution was
formed, which was then stored at ꢀ608C. Colorless crystals of 1 were
obtained after 24h. Yield: 0.15 g, 0.10 mmol, 30%. M.p. 68–738C
(decomp). 1H NMR (300 MHz, [D8]toluene, ꢀ408C): d = 0.62 (s, 6H;
InMe2), 0.70 ppm (s, 18H; SiMe3). 13C{1H} NMR (75 MHz, [D8]tol-
uene, ꢀ408C): d = ꢀ1.1 (InMe2), 6.8 ppm (SiMe3). MS (EI, 12 eV,
508C) m/z (%) = 428 (13) [Bi(SiMe3)3]+, 282 (13) [BiSiMe3]+, 145 (65)
[InMe2]+, 131 (100) [Si2Me5]+, 73 (39) [SiMe3]+.
2: A suspension of [{Me2AlP(SiMe3)2}2] (0.23 g 0.5 mmol) in
ꢀ
toluene (10 mL) at 08C was combined with [Me3Tl dmap] (0.37 g,
Figure 2. Solid-state structures of 2 (above) and 3 (below): Selected
bond lengths [Š] and angles [8]. 2: Tl1-P1 2.695(2), Tl1-P1A 2.688(2),
P1-Si1 2.246(2), P1-Si2 2.245(2), Tl1-C7 2.198(5), Tl1-C8 2.210(5); Tl1-
P1-Tl1A 95.5(1), P1-Tl1-P1A 84.5(1), C7-Tl1-C8 122.3(2), Si1-P1-Si2
109.0(1). 3: Tl1-As1 2.781(1), Tl1-As1A 2.743(1), As1-Si1 2.334(2), As1-
Si2 2.338(2), Tl1-C7 2.192(6), Tl1-C8 2.202(6); Tl1-As1-Tl1A 96.7(1),
As1-Tl1-As1A 83.3(1), C7-Tl1-C8 124.6(3), Si1-As1-Si2 108.5(1).
1.0 mmol) dissolved in toluene (5 mL). The resulting clear solution
was stirred for an additional 30 min and stored at ꢀ308C. Colorless
crystals of 2 were obtained after 24 h. Yield: 0.33 g, 0.28 mmol, 56%.
M.p. 107–1118C (decomp). 1H NMR (300 MHz, C6D6, 308C): d = 0.35
(s, 18H; SiMe3), 1.00 ppm (d, 2JTl-H = 317 Hz, 6H; TlMe2).
13C{1H} NMR (75 MHz, C6D6, 308C): d = 5.0 (s; SiMe3), 12.8 ppm
(d, 1JTl-C = 2391 Hz; TlMe2). 31P{1H} NMR (120 MHz, C6D6, 308C):
1
d = ꢀ234 ppm (t, JTl-P = 2462 Hz). MS (EI, 12 eV, 758C) m/z (%) =
557 (5) [(Me3Si)2PTlP(SiMe3)2]+, 416 (26) [P4(SiMe3)4]+, 401 (100)
[P4(SiMe3)3(SiMe2)]+, 395 (20) [MeTlP(SiMe3)2]+, 328 (12)
[P4(SiMe3)2(SiMe2)]+, 313 (24) [P4(SiMe3)(SiMe2)2]+, 203 (7) Tl+,
178 (36) [HP(SiMe3)2]+, 163 (5) [P(SiMe3)(SiMe2)]+, 73 (25) [SiMe3]+.
3: A suspension of [{Me2AlAs(SiMe3)2}2] (0.28 g, 0.5 mmol) in
¯
are isostructural and crystallize in the triclinic space group P1
(No. 2).[15] The central structural units of 2 and 3 are the four-
membered planar Tl2E2 rings. The Tl- and P- or As-atoms
each adopt distorted tetrahedral environments. To date, only
two Lewis acid–base adducts containing a TlIII center with a
ꢀ
toluene (10 mL) at ꢀ308C was combined with [Me3Tl dmap] (0.37 g,
1.0 mmol) dissolved in toluene (5 mL). The resulting slightly brown
suspension was stirred for 30 min, then warmed quickly until a clear
solution was formed, and finally stored at ꢀ608C. Colorless crystals of
3 were obtained after 24 h. Yield: 0.29 g, 0.32 mmol, 64%. M.p. 118–
ꢀ
ꢀ
Tl P bond have been structurally characterized. The Tl P s-
bond in 2 (2.688(1), 2.695(1) Š) is significantly shorter than
ꢀ
the dative Tl P bond observed in the intramolecularly
Angew. Chem. Int. Ed. 2003, 42, 5641 –5644
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1218C (decomp, darkening above 708C). 1H NMR (300 MHz,
[8] In addition to the binary intermetallic phases InxBiy only some
2
ꢀ
[D8]toluene, ꢀ108C): d = 0.32 (s, 18H; SiMe3), 0.97 ppm (d, JTl-H
=
Zintl anions are known with In Bi bond lengths ranging from
313 Hz, 6H; TlMe2). 13C{1H} NMR (75 MHz, [D8]toluene, ꢀ108C):
d = 4.3 (s; SiMe3), 12.4 ppm (d, 1JTl-C = 2304 Hz; TlMe2). MS (EI,
12 eV, 1508C) m/z (%) = 442 (76) [As2(SiMe3)4]+, 354 (11) [As2(SiM-
e3)2(SiMe2)]+, 339 (11) [As2(SiMe3)(SiMe2)2]+, 294 (30)
[(As(SiMe3)3]+, 266 (61) [As2(SiMe2)2]+, 233 (24) [TlMe2]+, 203 (9)
Tl+, 131 (15) [Si2Me5]+, 73 (100) [SiMe3]+.
2.83–3.15 Š. a) R. Kubiak, Z. Anorg. Allg. Chem. 1977, 431, 261;
b) L. Xu, S. C. Sevov, Inorg. Chem. 2000, 39, 5383; c) S. Bobev,
S. C. Sevov, J. Solid State Chem. 2002, 163, 436.
[9] A. F. Holleman, E. Wiberg, Lehrbuch der Anorganischen
Chemie, 101th ed., W. de Gruyter, Berlin, 1995, p. 1838.
[10] The formation of the heterocycles was unequivocally proven by
NMR spectroscopy.
Received: July 8, 2003 [Z52332]
[11] [{Me2AlBi(SiMe3)2}3] fails to react with InMe3 at the required
low-temperature conditions (ꢀ508C). Only upon the addition of
an equimolar amount of dmap is a fast and complete reaction
achieved.
[12] In THF, solvent-stabilized monomeric units [(iBu)2Al(thf)PH2]
are formed. M. Driess, C. Monse, Z. Anorg. Allg. Chem. 2000,
626, 1091.
[13] a) E. Hey-Hawkins, M. F. Lappert, J. L. Atwood, S. G. Bott, J.
Chem. Soc. Dalton Trans. 1991, 939; b) R. L. Wells, A. T.
McPhail, T. M. Speer, Eur. J. Solid State Inorg. Chem. 1992, 29,
63.
[14] Despite the presence of two NMR-active Tl isotopes, only a
single coupling is observed in the NMR spectra.
Keywords: Group 13 elements · Group 15 elements ·
.
heterocycles · main group element · metathesis
[1] See the following and references in: S. Schulz, Struct. Bonding
(Berlin) 2002, 103, 117.
[2] Several indium hydrides have been synthesized within the last
decade, whereas to the best of our knowledge molecular thallium
hydrides are unknown to date. S. Aldridge, A. J. Downs, Chem.
Rev. 2001, 101, 3305.
[3] S. Schulz, M. Nieger, Angew. Chem. 1999, 111, 1020; Angew.
Chem. Int. Ed. 1999, 38, 967.
[4] F. Thomas, T. Bauer, S. Schulz, M. Nieger, Z. Anorg. Allg. Chem.
2003, 629, 2018.
[5] Detailed studies on the reaction of base-stabilized compounds of
[15] 2: C16H48P2Si4Tl2, Mr = 823.58, colorless crystal 0.40 ” 0.30 ”
¯
0.20 mm; triclinic, space group P1 (No. 2); a = 9.2585(2), b =
9.5727(2), c = 9.6569(3) Š, a = 77.581(1), b = 81.497(1), g =
64.991(2)8, V= 755.92(3) Š3; Z = 1; m = 10.910 mmꢀ1; 1calcd
=
1.809 gcmꢀ3
; 10498 reflections (2qmax = 558), 3378 unique
ꢀ
the type [dmap Al(Me2)E(SiMe3)2] (E = P, Sb) with MMe3
(Rint = 0.049), 109 parameters; largest max./min. in the final
difference Fourier synthesis: 1.709 eŠꢀ3/ꢀ2.514 eŠꢀ3; max./
min. transmission 0.1589/0.0846; R1 = 0.029 (I > 2s(I)), wR2 =
0.072. 3: C16H48As2Si4Tl2, Mr = 911.48, colorless crystal 0.40 ”
(M = Al, Ga, In) demonstrate the initial formation of the
ꢀ
ꢀ
corresponding adducts [dmap Al(Me2)E(SiMe3)2 MR3], which
ꢀ
ꢀ
consequently undergo Al E bond breakage and M E bond
formation reactions. F. Thomas, S. Schulz, M. Nieger, Organo-
metallics 2003, 22, 3471.
¯
0.35 ” 0.30 mm; triclinic, space group P1 (No. 2); a = 9.2985(4),
b = 9.4604(3), c = 9.7580(4) Š, a = 78.332(2), b = 82.125(2), g =
[6] F. Thomas, S. Schulz, M. Nieger, Organometallics 2002, 21, 2793.
[7] Single-crystal X-ray diffraction: Nonius Kappa CCD diffrac-
tometer (MoKa radiation, l = 0.71073 Š; T= 123(2) K). The
structures were solved by direct methods (SHELXS-97, G. M.
Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467) and refined by
full-matrix least-squares on F2 (G. M. Sheldrick, SHELXL-97,
Program for Crystal Structure Refinement, Universität Göttin-
gen, Göttingen (Germany), 1997). All non-hydrogen atoms were
refined anisotropically and hydrogen atoms by a riding model.
66.191(2)8, V= 767.64(5) Š3; Z = 1; m = 12.779 mmꢀ1; 1calcd
1.972 gcmꢀ3; 6618 reflections (2qmax = 558), 3321 unique (Rint
=
=
0.055), 109 parameters; largest max./min. in the final difference
Fourier synthesis: 2.177 eŠꢀ3/ꢀ3.042 eŠꢀ3; max./min. transmis-
sion 0.1311/0.0639; R1 = 0.038 (I > 2s(I)), wR2 = 0.093. CCDC-
213920 (1), CCDC-213921 (2), and CCDC-213922 (3) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
Empirical
absorption
corrections
were
applied. 1:
C24H72Bi3In3Si6, Mr = 1500.76, colorless crystal, 0.45 ” 0.40 ”
0.35 mm; monoclinic, space group P2(1)/n (No. 14); a =
9.6601(2), b = 20.9419(4), c = 24.8825(5) Š, b = 96.354(1)8, V=
[16] a) G. Mꢀller, J. Lachmann, Z. Naturforsch. B 1993, 48, 1544;
b) R. A. Baldwin, R. L. Wells, P. S. White, Main Group Chem.
1997, 2, 67.
5002.83(17) Š3; Z = 4; m = 12.031 mmꢀ1
; ;
1calcd = 1.993 gcmꢀ3
41988 reflections (2qmax = 508), 8812 unique (Rint = 0.070), 325
[17] D. Huang, J. D. Corbett, Inorg. Chem. 1998, 37, 4006.
[18] F. Thomas, S. Schulz, M. Nieger, Z. Anorg. Allg. Chem. 2002,
628, 235.
parameters; largest max./min. in the final difference Fourier
; max./min. transmission
0.1084/0.0751; R1 = 0.035 (I > 2s(I)), wR2 = 0.065.
ꢀ3
synthesis: 1.068 eŠꢀ3/ꢀ1.366 eŠ
5644
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2003, 42, 5641 –5644